Innovation drive

13 October 2008 00:00[Source: ICB]

ExxonMobil Chemical and TonenGeneral Sekiyu have triumphed in the product innovation category, with coextruded films that bring electric cars closer to reality

THE QUEST to improve the fuel efficiency of cars and trucks takes in a variety of approaches. Among these today is the multi-faceted effort to build lithium-ion batteries equal to the task of expanding energy storage capacity, power and lifetime, as well as reducing weight and improving safety.

With their development of an advanced coextruded battery separator film (BSF), global major ExxonMobil Chemical and Japanese affiliate TonenGeneral Sekiyu are playing a key role in achieving these goals.

"ExxonMobil's BSF technology has successfully demonstrated critical technical properties and characteristics that are key to meeting increasingly demanding requirements for powering next-generation vehicles with smaller and lighter lithium-ion batteries," says Jim Harris, senior vice president at ExxonMobil Chemical. "This advancement will help put more hybrid and electric vehicles on the road, which means improved energy efficiency and fewer emissions."

The BSF also has applications in such portable power uses as notebook computers, power tools, uninterrupted power supplies, and motorbikes.

HOW IT WORKS

The charge and discharge cycles of a lithium-ion battery are accompanied by movement of lithium ions between the battery electrodes in an electrolyte. The ions must pass through a thin, microporous, electrically insulating membrane that separates the anode from the cathode.

The separator allows the battery to function efficiently and safely with low resistance to ion transport through very small, uniform pores that are about 100 nanometers wide.

"Getting the pores right is basically what allows the battery to breathe," says Patrick Brant, chief polymer scientist at ExxonMobil Chemical. "The more porous the separator is, while still retaining good toughness characteristics, the more work you can do with the electrons in the external circuit."

The separator, typically a highly engineered polyolefin film, should:

be chemically inert, uniform, free of flaws and strong

insulate the cathode from the anode

be permeable enough to allow ready ion transport, and

ideally, serve as a safety device by shutting down the flow of ions in the event of thermal abuse, and maintain this capability over a wide temperature range.

ExxonMobil and TonenGeneral started work in 2005 to bring together a host of technologies, including TonenGeneral's wet process fabrication technology and formulation know-how, and ExxonMobil's coextrusion and polymer design capabilities.

At Baytown, Texas, ExxonMobil developed high-heat resistance polymers tailored to TonenGeneral's process, while in Japan, TonenGeneral improved its film-making process, which combines multiple polymers into a single, 20 or 25 micrometer-thick porous sheet.

In a typical example, three layers are coextruded, with each layer designed for a different function. The core layer performance can be tailored with polyethylene (PE) and ultrahigh-molecular weight polypropylene (PP), while the skin layers can have other compositions. Such designs, says Brant, "help optimize permeability and thermal stability while retaining existing valued proprietary separator characteristics."

ExxonMobil has now commercialized four grades of its battery separator film for the demanding requirements of hybrid and electric vehicles. It will also tailor the design to meet specific OEM/battery manufacturer requirements, making lithium-ion battery adoption faster and easier.

"We are currently working with industry-leading battery manufacturers to expand the boundaries of current hybrid and electric vehicle applications," says Harris. For example, the company has collaborated with Canada-based Electrovaya, a developer and manufacturer of portable power solutions, to incorporate BSF into its proprietary Lithium Ion SuperPolymer battery system targeted at the automotive industry.

Electrovaya uses this battery system in the Maya-300, a zero-emission, low-speed electric vehicle with an extended range of up to 120 miles (193km). Electrovaya technology will also be used in plug-in hybrids from Visionary Vehicles and electric vehicles from Phoenix Motorcars, both US-based.

The separator film is produced at TonenGeneral's manufacturing plant in Nasu, Japan. A new TonenGeneral plant is being built in Gumi, South Korea, and will use advanced polymer and process technologies to produce superior quality and performance battery separator films.